1. Field of Invention
This invention is related to semiconductor structures including group III-V nitride films formed on c-face sapphire substrates.
2. Description of Related Art
High-efficiency, solid-state light-emitting devices based on group III-V nitrides (xe2x80x9cIII-V nitridesxe2x80x9d) have sufficiently wide bandgaps for short-wavelength, visible light emission. III-V nitrides have been used in light-emitting diode (LED) devices to provide bright green-to-ultraviolet light at high efficiencies. Known LEDs cover all three primary colors of the spectrum (red, green and blue). Accordingly, these devices can be used in various combinations to produce any color.
III-V nitrides have also been used to fabricate diode lasers that operate at room temperature and emit visible light in the blue-violet range under continuous operation. Diode lasers including III-V nitrides can emit coherent radiation and focus light into small spots, enabling high-density optical information storage and printing. Blue lasers are particularly promising due to their short wavelength. In addition, blue lasers can be combined with red and green lasers to create projection displays, color film printers and other devices.
Sapphire is a commonly used substrate material for growing III-V nitride films for light-emitting devices. A-face and c-face single crystal sapphire substrates are most commonly used for growing such III-V nitride films.
Diode laser devices include a resonator cavity having opposed mirror surfaces. When electricity passes through the device, photons are produced, which bounce back and forth inside the resonator cavity. The photons move together in phase and at the same wavelength in the resonator cavity, which increases the light intensity. The emitted light forms a narrow column of bright light at a single wavelength, i.e., coherent light.
In order to form diode lasers, facets are formed in the semiconductor structures formed on substrates. The facets form the mirror surfaces of the resonator cavity of the laser diode. It is possible to successfully etch through III-V nitride films formed on sapphire substrates. However, sapphire is difficult to etch. Consequently, portions of the sapphire substrate adjacent to the facets formed in the III-V nitride materials by etching would not be etched through by the etching process, and would interfere with light emitted from the laser diodes. Particularly, the top surface of the sapphire substrate would reflect the emitted light back and create interference fringes in the emitted light pattern. The resulting light emission pattern would not be an acceptable far-field pattern. Accordingly, the laser diodes would be unsuitable for applications that require an acceptable far-field pattern, such as printing applications.
Methods of cleaving sapphire substrates are known. The crystalline structure of sapphire is shown in FIG. 1. The sapphire unit cell 20 has a hexagonal crystal structure. The location and orientation of an a-plane 22, c-planes 24, an m-plane 26 and an r-plane 28 of the sapphire unit cell 20 are shown. The a-plane 22 has a (11{overscore (2)}0) orientation. The c-planes 24 form top and bottom c-faces of the sapphire unit cell 20 and have (0001) orientations. The a-plane 22 is perpendicular to the c-planes 24. The m-planes 26 form side surfaces of the sapphire unit cell 20 and have (1{overscore (1)}00) orientations. The r-plane 28 is oriented at an angle xcex1 of 57.6xc2x0 with respect to the c-planes 24.
The unit cell of GaN, a III-V nitride compound semiconductor, is shown in FIG. 2. The GaN unit cell 30 has a hexagonal crystal structure and includes m-planes 32 forming side faces, and c-planes 34 forming top and bottom c-faces, of the GaN unit cell 30.
Methods for cleaving facets for III-V semiconductor structures grown on a-face sapphire substrates are known. FIG. 3 shows an a-face sapphire substrate 40 including a c-plane flat 42. GaN films grown on a-face sapphire substrates have been cleaved by making a peck on the GaN surface, at an angle of 57.6 degrees with respect to the c-plane flat. A cleave is formed in a direction extending along the r-plane of the sapphire substrate and along an m-plane of GaN, as depicted by line 44.
FIG. 4 is a side view after cleavage of a GaN film 50 formed on the a-face sapphire substrate 40. The cleave is along the (1{overscore (1)}02) r-plane of the a-face sapphire substrate 40 and an m-plane of the GaN film 50.
However, there is a need for methods for forming facets for III-V nitride films grown on c-face sapphire substrates. There is also a need for methods for forming facets for light-emitting devices comprising III-V nitride films grown on c-face sapphire substrates. This invention satisfies these and other needs and provides methods for cleaving facets for III-V nitride films formed on c-face sapphire substrates. This invention separately provides methods for cleaving facets for light-emitting devices comprising such III-V nitride films, formed on c-face sapphire substrates.
This invention also provides light-emitting devices comprising III-V nitride films formed on c-face sapphire substrates.
An exemplary embodiment of the methods for cleaving facets for III-V nitride films according to this invention comprises forming a semiconductor structure comprising at least one III-V nitride layer on the top c-face of a c-face sapphire substrate, which also includes a bottom c-face. A line of weakness is formed on the bottom c-face of the c-face sapphire substrate. The line of weakness extends in the a-plane direction of the c-face sapphire substrate. A force is applied to the bottom c-face to cleave the c-face sapphire substrate along the line of weakness in the a-plane direction, and to form a cleaved facet along an m-plane of each III-V nitride layer.
According to the invention, the semiconductor structures can include laser diodes and light-emitting diodes.